I believe that the 8 million number is not taking into account the Top 51 rule.
The Pats were under the cap by 10.8 million on March 6th. Since then they signed Lewis Sanders, Tank Williams, and Fernando Bryant to vet min deals meaning that the 3 displaced players with $370,000 salaries from the Top 51 list so the net effect of signing the trio was less than $400,000 (490,000 - 370,000)*3.
I have the Pats under the cap by 10.5 million.
There are 8 players not in the Top 51 list.
5 of them have $370,000 salaries
3 of them have $295,000 salaries.
1,850,000 (370,000 *5)
885,000 (295,000 *3)
2,735,000
10.5 million minus 2,735,000 is approximately 8 million.